1. Field of the Invention
This invention relates to a radiation image recording and read-out apparatus wherein a radiation image is stored on each of different portions of a stimulable phosphor sheet, each portion of the stimulable phosphor sheet is then exposed to stimulating rays, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to radiation, and the emitted light is detected and converted into an electric image signal representing the whole radiation image. This invention particularly relates to a radiation image recording and read-out apparatus wherein an image signal, which represents the whole radiation image and which has a high signal-to-noise ratio (S/N ratio), is obtained or an image signal corresponding to a specific structure in a radiation image, i.e. to only a certain part of the whole radiation image, is obtained.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy which was stored. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation, which has passed through an object, such as a human body. In this manner, a radiation image of the object is stored on the stimulable phosphor sheet. The stimulable phosphor sheet, on which the radiation image has been stored, is then scanned with stimulating rays, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, when it is exposed to the stimulating rays, is photoelectrically detected and converted into an electric image signal. The electric image signal is then processed as desired, and the processed image signal is then used during the reproduction of a visible image which has good image quality and can serve as an effective tool, such as the efficient and accurate diagnosis of an illness. The visible image finally obtained may be reproduced in the form of a hard copy or may be displayed on a display device, such as a cathode ray tube (CRT) display device. In the radiation image recording and reproducing systems, the stimulable phosphor sheet is used to store the radiation image temporarily so that a final visible image can be reproduced therefrom on a final recording medium. For the sake of economy, therefore, it is desirable that the stimulable phosphor sheet be used repeatedly.
In order that the stimulable phosphor sheets may be reused as described above, the energy remaining on the stimulable phosphor sheet after it has been scanned with stimulating rays should be erased. For this purpose, the stimulable phosphor sheet may be exposed to light or heat as described in, for example, U.S. Pat. No. 4,400,619 or Japanese Unexamined Patent Publication No. 56(1981)-12599. The stimulable phosphor sheet may then be used again for the recording of a radiation image.
Techniques for carrying out superposition processing on radiation images have heretofore been disclosed in, for example, U.S. Pat. No. 4,356,398. In general, radiation images are used for diagnoses of illnesses and for other purposes. When a radiation image is used for such purposes, it is required that even small differences in the radiation energy absorption characteristics among structures of an object can be detected accurately in the radiation image. The extent, to which such differences in the radiation energy absorption characteristics can be detected in a radiation image, is referred to as the contrast detection performance or simply as the detection performance. A radiation image having better detection performance has better image quality and can serve as a more effective tool particularly in, the efficient and accurate diagnosis of an illness. Therefore, in order for the image quality to be improved, it is desirable that the detection performance of the radiation image may be improved. The detection performance is adversely affected by various noises.
In radiation image recording systems using stimulable phosphor sheets, it has been found that the noises described below occur during the step of recording a radiation image on a stimulable phosphor sheet and reading out the radiation image therefrom.
(1) A quantum noise of radiation produced by a radiation source.
(2) A noise due to nonuniformity in the distribution of the stimulable phosphor coated on the stimulable phosphor sheet or in the distribution of the stimulable phosphor grains on the stimulable phosphor sheet.
(3) A noise from stimulating rays, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to radiation.
(4) An electric noise in the means for detecting light emitted by the stimulable phosphor sheet and converting it into an electric signal.
(5) A noise of light emitted by the stimulable phosphor sheet.
Superposition processing is carried out in order to reduce the aforesaid noises markedly so that even small differences in the radiation energy absorption characteristics among structures of an object can be found accurately in a visible radiation image, which is reproduced finally, i.e. the detection performance of the radiation image can be improved markedly. General techniques for superposition processing and its effects are described below.
A radiation image is stored on each of a plurality of stimulable phosphor sheets, which have been placed one upon another. Thereafter, an image read-out operation is carried out for each of the stimulable phosphor sheets. A plurality of image signals, which have been obtained from the image read-out operations, are superposed one upon another. In this manner, various noises described above can be reduced. Specifically, in general, noises described in (1) through (5) exhibit different distributions for different radiation images stored on the stimulable phosphor sheets. When the image signals detected from the stimulable phosphor sheets are superposed one upon another, the noises can be averaged. Therefore, the noises become imperceptible in a superposition image, which is obtained from superposition processing. Specifically, an image signal having a high S/N ratio is obtained from superposition processing. More specifically, most of the noises described in (1) through (5), particularly, the noise described in (1), which is one of dominant factors among the noises in a radiation image, can be approximated by the Poisson statistics. In cases where noises can be approximated by the Poisson statistics and two radiation images yield equivalent levels of signals S1 and S2 and equivalent levels of noises N1 and N2, the level of the signal corresponding to a superposition image equals S1+S2. The superposition image is obtained by carrying out superposition processing on the two radiation images. The level of noise in the superposition image becomes equal to .sqroot.N1.sup.2 +N2.sup.2. The S/N ratio is one of the indexes representing the detection performance of a radiation image. The S/N ratios of the two radiation images prior to superposition processing are represented by the formulas S1/N1 and S2/N2. After superposition processing has been carried out on the two radiation images, the S/N ratio of the superposition image is represented by the formula (S1+S2)/.sqroot.N1.sup.2 +N2.sup.2. Therefore, as a result of superposition processing, the S/N ratio can be improved. When superposition processing is carried out on image signals representing the two radiation images, the values of the image signals may be weighted such that a markedly high S/N ratio can be obtained.
When a visible radiation image is to be reproduced from the image signal obtained from superposition processing, gradation processing should preferably be carried out in order to improve the contrast of the image. In such cases, the contrast of the whole image may be improved. Alternatively, the contrast may be improved only for specific frequency components, i.e. frequency response enhancement processing may be carried out. As another alternative, both the processing for improving the contrast of the whole image and the frequency response enhancement processing may be carried out. When the values of a plurality of image signals are added together or averaged during superposition processing, an image signal detected from a stimulable phosphor sheet, which is located closer to the radiation source than the other stimulable phosphor sheets are, should preferably be weighted with a larger weighting coefficient. In this manner, a better superposition image can be obtained than when the values of the respective image signals are merely added or averaged. Appropriate weighting coefficients vary, depending on the number of the stimulable phosphor sheets, which are placed one upon another during the recording of the radiation images, the thicknesses of the stimulable phosphor sheets, or the like.
By way of example, when superposition processing is to be carried out, two stimulable phosphor sheets have heretofore been housed in a cassette such that they overlap one upon the other. Radiation images of an object are then recorded on the two stimulable phosphor sheets housed in the cassette. Thereafter, an image read-out operation is carried out on each of the two stimulable phosphor sheets, and two image signals are thereby obtained.
Also, techniques for carrying out subtraction processing on radiation images have heretofore been known. When subtraction processing is to be carried out, two radiation images recorded under different conditions are photoelectrically read out, and digital image signals which represent the radiation images are obtained. The image signal components of the digital image signals which represent corresponding picture elements in the radiation images are then subtracted from each other, and a difference signal is thereby obtained which represents the image of a specific structure or part of the object represented by the radiation images. With the subtraction processing method, two digital image signals are subtracted from each other in order to obtain a difference signal, and the radiation image of a specific structure can be reproduced from the difference signal.
Basically, subtraction processing is carried out with either the so-called temporal (time difference) subtraction processing method or the so-called energy subtraction processing method. In the former method, in order to extract the image of a specific structure of an object from the image of the whole object, the image signal representing a radiation image obtained without injection of contrast media is subtracted from the image signal representing a radiation image in which the image of the specific structure of the object is enhanced by the injection of contrast media. In the latter method, an object is exposed several times to radiation with different energy distributions, or the energy distribution of the radiation, which has passed through an object, is changed after it has been irradiated onto one of two radiation storage means, after which the radiation impinges upon the second storage means. In this manner, two radiation images, in which different images of a specific structure are embedded, are obtained. Thereafter, the image signals representing the two radiation images are weighted appropriately, when necessary, and subjected to a subtraction process in order to extract the image of the specific structure.
Subtraction processing is extremely effective, particularly for medical diagnosis, and electronics research has continued to develop improved subtraction processing methods.
In the aforesaid radiation image recording and reproducing systems utilizing a stimulable phosphor sheet, the radiation image stored on the stimulable phosphor sheet is read out directly as an electric image signal. Therefore, with such radiation image recording and reproducing systems, the aforesaid subtraction processing can readily be carried out. In cases where energy subtraction processing is to be carried out, radiation images may be stored on two stimulable phosphor sheets so that the parts of the radiation images corresponding to a specific structure are different in the two radiation images. For this purposes, two-shot energy subtraction processing may be employed wherein the operation for recording a radiation image is carried out twice with two kinds of radiation having different energy distributions. Alternatively, one-shot energy subtraction processing may be employed wherein, for example, two stimulable phosphor sheets placed one upon the other are simultaneously exposed to radiation, which has passed through an object, such that they are exposed to radiation having different energy distributions.
In order to carry out one-shot energy subtraction processing, the following methods have been proposed:
(1) A method wherein a filter, which is constituted of a metal or the like and which absorbs low energy components of radiation, is located between two stimulable phosphor sheets, and radiation having different energy distributions is thereby obtained.
(2) A method wherein two stimulable phosphor sheets provided with layers of different types of stimulable phosphors are utilized so that no filter need be used and radiation images to be subjected to subtraction processing can be recorded with a single image recording operation. With this method, a stimulable phosphor sheet provided with a stimulable phosphor layer, which absorbs more of the low energy components of the radiation than the stimulable phosphor layer of the other stimulable phosphor sheet, is positioned closer to the object (closer to the radiation source), and the two stimulable phosphor sheets are simultaneously exposed to radiation. Such a method is disclosed in, for example, U.S. Pat. No. 4,855,598.
However, in cases where radiation images are recorded on a plurality of stimulable phosphor sheets, which are placed one upon another, the stimulable phosphor sheets are housed in a cassette and subjected to the image recording operation. After the radiation images have been stored on the stimulable phosphor sheets but before they are read out from the stimulable phosphor sheets, the stimulable phosphor sheets must be taken out of the cassette and respectively loaded into new independent cassettes, so that the image read-out operation can be carried out on each of the stimulable phosphor sheets. Therefore, troublesome operations and considerable time are required.
Also, when the plurality of the stimulable phosphor sheets are sequentially subjected to the image read-out operations, the time taken for the image read-out operations becomes long.
As described above, though superposition processing and energy subtraction processing are efficient for diagnoses, the conventional techniques for superposition processing and energy subtraction processing have the drawbacks in that troublesome operations and considerable time are required. Therefore, it has heretofore not always been possible to carry out superposition processing or energy subtraction processing, particularly during mass medical examinations, or the like.
Accordingly, in Japanese Patent Application No. 1(1989)-53179, the applicant has proposed a novel radiation image recording and read-out apparatus. With the proposed radiation image recording and read-out apparatus, superposition processing or energy subtraction processing (specifically, one-shot energy subtraction processing utilizing no radiation energy converting filter) is carried out with a stimulable phosphor sheet such that troublesome operations for, for example, taking out stimulable phosphor sheets from a cassette need not be carried out and image read-out operations may be carried out quickly.
Specifically, in the proposed radiation image recording and read-out apparatus, two long strip-shaped, flexible stimulable phosphor sheets are located in parallel, and radiation images are recorded on the two stimulable phosphor sheets with a single, simultaneous exposure to radiation. Also, two independent image read-out sections for reading out the radiation images stored the stimulable phosphor sheets and two independent erasing sections for erasing any energy remaining on the stimulable phosphor sheets are provided for the two stimulable phosphor sheets, so that the two radiation images may be read out simultaneously.
With the proposed radiation image recording and read-out apparatus, no cassette is used, and superposition processing or energy subtraction processing can be carried out quickly and easily. Particularly, the proposed radiation image recording and read-out apparatus is advantageous for mass medical examinations, or the like, wherein quick processing is required.
However, with the proposed radiation image recording and read-out apparatus, two stimulable phosphor sheets are used. Therefore, two image read-out sections are necessary for reading out the radiation images from the two stimulable phosphor sheets. Accordingly, the radiation image recording and read-out apparatus has the drawbacks in that it becomes large and complicated.